Phase-separated thin film structures for efficient polymer blend light-emitting diodes.

We report laterally and vertically phase-separated thin film structures in conjugated polymer blends created by polymer molecular weight variation. We find that micrometer-scale lateral phase separation is critical in achieving high initial device efficiency of light-emitting diodes, whereas improved balance of charge carrier mobilities and film thickness uniformity are important in maintaining high efficiency at high voltages. The optoelectronic properties of these blend thin films and devices are strongly influenced by the polymer chain order/disorder and the interface state formed at polymer/polymer heterojunctions.

Optoelectronic and charge transport properties at organic-organic semiconductor interfaces: comparison between polyfluorene-based polymer blend and copolymer.

We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT [f8-bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.

Uniaxial alignment of liquid-crystalline conjugated polymers by nanoconfinement.

We demonstrate the uniaxial alignment of a liquid-crystalline conjugated polymer, poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) by means of nanoconfinement during nanoimprinting. The orientation of the conjugated backbones was parallel to the nanolines imprinted into the polymer film. Polarized UV-vis absorption and photoluminescence spectra were measured to quantify the degree of alignment, showing that the polarization ratio and uniaxial molecular order parameter were as high as 66 and 0.97, respectively. The aligned F8BT film was used as the active layer in a PLED, which resulted in polarized electroluminescence with a polarization ratio of 11. Ambipolar PFET in a top-gate configuration with aligned F8BT as the active semiconducting layer showed mobility enhancement when the chains were aligned parallel to the transport direction. Mobility anisotropies for hole and electron transport were 10-15 and 5-7, respectively, for current flow parallel and perpendicular to the alignment direction.

Anisotropic optical properties in electroluminescent conjugated polymers based on grazing angle photoluminescence measurements.

Grazing angle photoluminescence (GPL) originates from a waveguided light emitted at grazing angle to the substrate due to the total internal reflections, and the light emission is polarized with enhanced intensity at selective mode wavelength. GPL measurements reveal the optical anisotropy of luminescent conjugated polymers, in particular, the alignment of emitting dipoles from which emission occurs, in contrast to spectroscopic ellipsometry measurements that give the anisotropy in the absorption. Based on the GPL emission intensities and spectra, we investigate the anisotropic optical properties in electroluminescent poly(9,9'-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer thin films of different molecular weights (M(n) = 9-255 kg/mol), both in the pristine and annealed states. The optical anisotropy in F8BT films generally increases with molecular weight, suggesting that higher molecular weight polymers with longer chains are more likely to lie in-plane to the substrate. Upon annealing, high molecular weight F8BT films show even a higher degree of anisotropy, in contrast to low molecular weight F8BT films that become more isotropic. Annealing causes the polymer chains to rearrange and adopt a configuration in which the interchain exciton migration to better ordered low energy (LE) emissive states is strongly suppressed. We observe that the emissive states in F8BT are strongly affected by the local polymer chain arrangement, producing the less ordered high energy (HE) emissive states near the substrate interface where there is a higher degree of chain disorder and the LE states in the bulk of the film. When spin coated onto a quartz substrate precoated with a poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) layer, films of F8BT show severe luminescence quenching near the PEDOT:PSS interface for both the LE and HE emissive states, but a selective quenching of the LE states in the bulk of the film. These observations have important implications for fabricating efficient electronic devices using conjugated polymers as an active material, since the performance of these devices will strongly depend on anisotropic optical properties of electroluminescent conjugated polymers.